The present invention relates to a liquid metal-cooled nuclear reactor, provided with a system for removing the residual power.
More specifically the invention relates to a novel system for removing the residual power in the case of a fast neutron reactor cooled by a liquid metal (generally liquid sodium), whereby said reactor is of the integrated type.
It is known that under normal operating conditions in liquid metal-cooled nuclear reactors the power given off by the fission reaction in the reactor core is absorbed by the so-called primary liquid metal circulation. The primary circuit is either located in the reactor vessel in the case of an integrated reactor or it passes out of the vessel in the case of a looped reactor. In most cases (and in all cases when the reactor is integrated) this liquid metal primary circuit is associated with a further liquid metal secondary circuit. This second circuit exchanges its heat with a water-vapour circuit which operates the turbines of the electrical power generation plant.
In addition to this systems for removing heat for supplying electric power reactors of this type comprise a cooling system, called the residual power removal system.
In the case of the French Superphenix reactor (liquid sodium-cooled integrated fast reactor) the residual power is removed in the following manner:
In the case of normal stoppages by starting-stopping circuits of the electricity generating system.
In the case of long stoppages or in the case of an electric power failure by sodium-air exchangers arranged in parallel on the liquid sodium secondary cooling circuits.
In the case where these normal residual power removal means do not exist all that remains are the following standby cooling means:
last resort cooling loops incorporating elongated generally cylindrical exchangers called "bottle" exchangers which are directly immersed in the liquid sodium of the primary vessel and are themselves connected to the sodium-air exchangers.
last resort cooling loops which are constituted by water circuits covering the shaft of the concrete-covered block in which is placed the main vessel and which receive the heat from the vessel by radiation.
It is added that in normal operation the wall of the main vessel is cooled by a tapped liquid sodium flow from the bases of the assemblies constituting the reactor. This flow then circulates in the annular space between the main reactor vessel and a thermal baffle cladding the main vessel.
The present invention relates to a system for removing the residual power from an integrated nuclear reactor cooled by a liquid metal which, on the one hand fulfils the functions normally fulfilled by such a system, but which also makes it possible to obtain the following advantages:
(a) the system according to the invention makes it possible to improve the cooling of the main vessel compared with the prior art cooling systems. If in particular the case is envisaged where the control rods do not drop it minimises the extraction of the latter from the core, said extraction being due to the differential heating of the main vessel compared with the control rod pins.
(b) The system according to the invention permits a better recovery of the molten fuel in the case of an accident leading to the at least partial fusion of the fuel material of the assemblies.
In a Superphenix reactor it is known that the assemblies are fitted by their bases into a support, which ensures both the positioning and the supply of the core with coolant. Between the support and the bottom of the main vessel there is a structure for recovering molten fuel in the case of an accident. In view of the melting temperature of the fuel (approximately 2,800.degree. C. in the case of uranium oxide) and the high residual power which it gives off it is obviously very important to cool the molten metal in this recovery structure to obviate other accidents. However, in the case of last resort cooling systems comprising bottle exchangers located substantially above the core level, they are only able to ensure a mediocre cooling of the molten fuel material. Thus, the distance between these exchangers and the recovery zone is relatively large. Furthermore, tightly sealed structures separate the volume of the "cold" collector where the recovery means are located from the volume of the "hot" collector in which are immersed the bottle exchangers. Under these conditions the cooling, which is only as a result of natural convection in the liquid sodium and by conduction through the metal walls may be inadequate. However, with the system according to the invention a better cooling of the recovery zone is obtained.